This invention relates to the disproportionation of alkylaromatic feedstreams and more particularly to disproportionation of toluene containing feedstocks employing dealuminated nickel mordenite catalysts.
Toluene disproportionation (TDP) involves a well known transalkylation reaction in which toluene is converted to benzene and xylene in accordance with the following reaction: 
Mordenite is one of a number of molecular sieve catalysts useful in the transalkylation of alkylaromatic compounds. Mordenite is a crystalline aluminosilicate zeolite exhibiting a network of silicon and aluminum atoms interlinked by oxygen atoms within the crystalline structure. For a general description of mordenite catalysts, reference is made to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, 1981, under the heading xe2x80x9cMolecular Sievesxe2x80x9d, Vol. 15, pages 638-643. Mordenite as found in nature or as synthesized to replicate the naturally occurring zeolite, typically exhibits a relatively low silica-to-alumina mole ratio of about 10 or less. Also known, however, are mordenite catalysts exhibiting a substantially lower alumina content. These alumina deficient mordenite catalysts exhibit silica-to-alumina ratios greater than 10, ranging up to about 100, and may be prepared by direct synthesis as disclosed, for example, in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies et al. Both the typical and the aluminum deficient mordenites are known to be useful in the disproportionation of toluene.
Disproportionation of toluene feedstock may be performed at temperatures ranging from about 200xc2x0 C. to about 600xc2x0 C. or above and at pressures ranging from atmospheric to perhaps 100 atmospheres or above and at liquid hourly space velocities (LHSV) of around 2 hrxe2x88x922. The specific catalyst, however, may impose constraints on reaction temperatures in terms of catalyst activity and aging characteristics. In general, the prior art suggests the use of relatively high temperatures when employing the high aluminum mordenites (low silica-to-alumina ratios) and somewhat lower temperatures when employing the low alumina mordenites. Accordingly, where mordenite catalysts exhibiting high silica/alumina ratios have been employed in the transalkylation of alkylaromatics, it has been the practice to operate toward the lower end of the temperature range. U.S. Pat. No. 4,665,258 to Butler et al., however, discloses disproportionation of a toluene containing feedstock employing an aluminum deficient mordenite catalyst under relatively severe disproportionation conditions; involving a temperature range of 370xc2x0-500xc2x0 C. The mordenite catalysts exhibit silica/alumina mole ratios of at least 30 and, more desirably, within the range of 40-60. The feedstock may be supplied to the reaction zone containing the mordenite catalyst at rates providing relatively high space velocities. The toluene weight hourly space velocity (WHSV) may be greater than 1. Hydrogen is supplied to the reaction zone at a hydrogen/toluene mole ratio within the range of 3-6. The hydrogen pressure may be 500 psi or more. The toluene feedstock need not be dried before supplying it to the reaction zone and the patent discloses toluene feedstocks exhibiting a water content in excess of 100 ppm.
The Butler ""258 patent also discloses passing a hot preflush gas, nitrogen or hydrogen, to the reaction zone prior to initiating the disproportionation reaction. The preflush gas is heated to a temperature sufficient to substantially dehydrate the catalyst by the time the toluene feed is started. This measure enables the disproportionation process to initially be performed at a somewhat lower temperature and without reduction in toluene conversion. As the disproportionation proceeds, temperature progressively increases to maintain toluene conversion at the desired level, typically about 80 percent of theoretical.
U. S. Pat. No. 4,723,049 to Menard et al. discloses toluene disproportionation carried out over aluminum deficient mordenite of the type disclosed in the aforementioned patent to Butler. In this process, preferably carried out at a reaction zone temperature of 370xc2x0-500xc2x0 C., and more preferably at a temperature of 400xc2x0-500xc2x0 C. with an unmodified aluminum deficient mordenite catalyst, the supply of toluene to the reaction zone is interrupted while the supply of hydrogen is continued. Preferably the period of interruption during which hydrogen supply is continued is for at least one day prior to reinstating the supply of toluene feedstock to the reaction zone. This mode of operation is disclosed to enhance the aging quality of the catalyst and show a reduction in reaction zone temperature without a corresponding decrease in toluene conversion.
It is also a common practice to promote an aluminum deficient mordenite catalyst with a catalytically active metallic content. For example, U.S. Pat. No. 3,476,821 to Brandenburg et al. discloses disproportionation reactions employing mordenite catalysts having silica/alumina ratios within the range of 10-100 and preferably within the range of 20-60. The mordenites are modified by the inclusion of a sulfided metal selected from the Group VIII metals. The metal may be included in the mordenite by well known ion exchange or impregnated techniques. The especially preferred sulfided Group VIII metals are cobalt and nickel present in a concentration of 0.5-10 weight percent. When compared with nickel oxide, nickel sulfide is said to provide less overactivity as indicated by gas and saturated hydrocarbon yield. Here the desired temperature ranges are said to be from about 400xc2x0-750xc2x0 F. and preferably 450xc2x0-640xc2x0 F. The metal promoters are said to substantially increase activity and catalyst life, as indicated by runs extending over several hours or days.
As noted previously, hydrogen is supplied along with toluene to the reaction zone. While the disproportionation reaction (1) does not involve chemical consumption of hydrogen, the use of a hydrogen cofeed is generally considered to prolong the useful life of the catalyst, as disclosed, for example, in the above mentioned patent to Brandenburg. The amount of hydrogen supplied, which is normally measured in terms of the hydrogen/toluene mole ratio, is generally shown in the prior art to increase as temperature increases.
Bhavikatti et al., xe2x80x9cToluene Disproportionation Over Aluminum-Deficient and Metal-Loaded Mordenites. 1. Catalytic Activity and Agingxe2x80x9d, Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 102-105, discloses toluene disproportionation at 400xc2x0 C. over mordenite catalysts having silica/alumina mole ratios ranging from 12 to 61 at atmospheric pressure and a space velocity (WHSV) of 1. As the silica/alumina mole ratio is increased, catalyst activity is substantially decreased while aging quality is increased. That is, the aging rates were lower. Based upon short term aging studies, the best silica/alumina mole ratio appeared to be 23. Catalyst decay was also suppressed by loading the mordenites with nickel. Mordenites having a silica/alumina ratio of 12, 16 and 23 were modified by the inclusion of nickel by a procedure involving ion exchanging ammonium mordenite with an aqueous solution of nickel nitrate. After ion exchange, the catalyst was activated under a hydrogen environment for two hours. The best activation temperature for nickel modified mordenite having a silica/alumina ratio of 23 was indicated to be about 550xc2x0 C. The nickel modified mordenite having a silica/alumina ratio of 12 showed significantly lower activity when compared to the nickel loaded mordenite of a silica/alumina ratio of 23.
Other patents directed to toluene disproportionation catalysts and processes include Mitsche U.S. Pat. No. 3,562,345; Mitsche U.S. Pat. No. 3,677,973; Marcilly U.S. Pat. No. 4,151,120; Dufresne et al. U.S. Pat. No. 4,723,048; and Pollitzer U.S. Pat. No. 3,780,122.
A pre-reaction start-up procedure comprising passing a hot, inert gas (hydrogen or nitrogen) across the catalyst and reactor bed prior to feedstock introduction is disclosed in Butler et al., U.S. Pat. No. 4,956,511 and U.S. Pat. No. 4,665,258 also to Butler et al. Another pre-reaction, start-up procedure aimed at controlling the hygroscopic tendency of mordenite involves subjecting the catalyst to a dry calcination procedure as disclosed in U.S. Pat. No. 4,151,120 to Marcilly. All of the aforementioned U.S. patents and literature references are incorporated herein by reference.
Conventional TDP processes utilizing Ni-Mordenite catalysts suffer from numerous disadvantages, including their sensitivity and susceptibility to being traumatized by ammonia, moisture, temperature changes, plant power failures, and other changes in operating conditions. Also, conventional Ni-mordenite catalysts suffer from the disadvantage of requiring up to three or more days to get lined out during reactor setup, during which period of startup, there is no significant acceptable production across the catalyst. The present invention provides a TDP process utilizing a Ni-mordenite catalyst that overcomes these deficiencies while allowing the production rates to be increased several fold and the selectivities to be improved at the same time.
In accordance with the present invention, there is provided a process for the disproportionation of a substantially pure toluene feedstock over a dealuminated nickel-mordenite catalyst wherein increased production rates of up to three times normal commercial rates can be achieved without detrimental effect on the product stream, catalyst activation, or catalyst life.